1. Field of the Invention
The present invention relates to a system for scanning a remote object, such as the earth, from an aircraft or satellite and, more particularly, to an improvement over a scanning system disclosed in an H. V. Kennedy U.S. Pat. No. 3,211,046, dated Oct. 12, 1965.
2. Description of the Prior Art
In FIG. 4 of the above-referred to Kennedy patent, a scanning system is shown which employs a rotating triangularly arranged mirror to receive radiation generated from a remote object, such as the earth, and to transmit that radiation to a plurality of folding mirrors and to a parabolic mirror where the energy is focused onto a radiation detector for purposes of producing an output indicative of the radiation received from various parts of the remote object. The outputs from the detector may be used for producing a picture or map of the object being scanned.
The Kennedy system has the problem of permitting stray radiation, ghost image radiation and narcissus reflection (described below) to reach the detector at unwanted times, and thereby cause an erroneous output to be produced. A narcissus reflection permits the detector, which in infrared systems is surrounded by a dewar which is at a very cold temperature, to "see" itself, i.e. a path from the detector to the parabolic mirror, through the folding mirrors and the rotating mirror, back to the parabolic mirror and then to the detector is provided. This is undesirable except once during calibration, because the very low temperature of the dewar around the detector is reflected back to the detector to produce a false reading. Ghost radiation is radiation other than that from the area of the remote object which is presently under observation and which finds a path to the detector which is generally parallel to the radiation from the area and thus appears, falsely, to come from the area. Stray radiation is all other radiation reaching the detector other than that from the area under observation. All narcissus, stray and ghost radiation adversely affect the output of the detector and are undesirable.